This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present invention. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present invention. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Emulsions, both oil-in-water (o/w) and water-in-oil (w/o), are commonly used in a range of applications, for example, foods, paints, cosmetics, lotions, and medications. The stability of such emulsions to shearing and aging can be critical to the performance of the products and their shelf life. An emulsion with poor stability may result in the rupture of the internal-phase droplets, thus forming a free phase. Free phase formation can reduce the texture and effectiveness of the product. Emulsion stability is typically enhanced by use of surface-active additives (e.g., surfactants). However, in certain cases it is desirable to utilize little or no additives to reduce cost or to avoid interference with other properties of the desired emulsion.
One useful application of emulsions is in the recovery of hydrocarbons from subterranean formations. Oil recovery is usually inefficient in subterranean formations (hereafter simply referred to as formations) where the mobility of the in situ oil being recovered is significantly less than that of the drive fluid used to displace the oil. Mobility of a fluid phase in a formation is defined by the ratio of the fluid's relative permeability to its viscosity. For example, when waterflooding is applied to displace very viscous heavy oil from a formation, the process is highly inefficient because the mobility of the viscous oil is much lower than the mobility of the water. The water quickly channels through the formation to the producing well, bypassing most of the oil and leaving it unrecovered. Consequently, there is a need to either make the water more viscous, or use another drive fluid that will not channel through the oil. Because of the large volumes of drive fluid needed, it must be inexpensive and stable under formation flow conditions. Oil displacement is most efficient when the mobility of the drive fluid is less than the mobility of the oil, so the greatest need is for a method of generating a low-mobility drive fluid in a cost-effective manner.
For modestly viscous oils—those having viscosities of approximately 10-300 centipoise (cp) water-soluble polymers such as polyacrylamides or xanthan gum have been used to increase the viscosity of the water injected to displace oil from the formation in a waterflooding operation. In this process, the polymer is dissolved in the water, increasing its viscosity. While such water-soluble polymers can be used to achieve a favorable mobility, it is not generally viable for higher viscosity oils (e.g., above 300 cp). These oils are so viscous that the amount of polymer needed to achieve a favorable mobility ratio would usually be uneconomic. Further, polymer dissolved in water often is adsorbed from the drive water onto surfaces of the formation rock, entrapping it and rendering it ineffective for viscosifying the water. This leads to loss of mobility control, poor oil recovery, and high polymer costs. For these reasons, use of polymer floods to recover oils in excess of about 300 cp is not usually economically feasible. Also, performance of many polymers is adversely affected by levels of dissolved ions typically found in formation brine, placing limitations on their use and/or effectiveness.
Water-in-oil macroemulsions (hereafter referred to simply as “emulsions” or “w/o emulsions”) have been proposed as a method for producing viscous drive fluids that can maintain effective mobility control while displacing moderately viscous oils. For example, the use of water-in-oil and oil-in-water macroemulsions have been evaluated as drive fluids to improve oil recovery of viscous oils. Although generally not discussed herein, microemulsions (i.e., thermodynamically stable emulsions) have also been proposed as flooding agents for hydrocarbon recovery from reservoirs, which may also be referred to as “emulsion flooding.”
Macroemulsions used for hydrocarbon recovery have been created by addition of sodium hydroxide to acidic crude oils from Canada and Venezuela. See, e.g., H. MENDOZA, S. THOMAS, and S. M. FAROUQ ALI, “Effect of Injection Rate on Emulsion Flooding for a Canadian and a Venezuelan Crude Oil”, Petroleum Society of CIM and AOSTRA 1991 Technical Conference (Banff, Alberta), Paper 91-26; and M. FIORI and S. M. FAROUQ ALI, “Optimal emulsion design for the recovery of a Saskatchewan crude,” Journal of Canadian Petroleum Technology, 30(2), 123-132, March-April 1991. These emulsions were stabilized by soap films created by saponification of acidic hydrocarbon components in the crude oil by sodium hydroxide. The soap films reduced the oil/water interfacial tension, acting as surfactants to stabilize the water-in-oil emulsion. It is well known, therefore, that the stability of such emulsions substantially depends on the use of caustic (e.g., sodium hydroxide) for producing a soap film to reduce the oil/water interfacial tension.
Various studies on the use of caustic for producing such emulsions have demonstrated technical feasibility. However, the practical application of this process for recovering oil has been limited by the high cost of the caustic, likely adsorption of the soap films onto the formation rock leading to gradual breakdown of the emulsion, and the sensitivity of the emulsion viscosity to minor changes in water salinity and water content. For example, because most formations contain water with many dissolved solids, emulsions requiring fresh or distilled water often fail to achieve design potential because such low-salinity conditions are difficult to achieve and maintain within the actual formation. Ionic species can be dissolved from the rock and the injected fresh water can mix with higher-salinity resident water, causing breakdown of the low-tension stabilized emulsion.
Bragg et al., (U.S. Pat. Nos. 5,855,243, 5,910,467, 5,927,404, 6,068,054) describes using a high water-cut water-in-oil emulsion stabilized with microparticles and diluted with dissolved gas to displace viscous oils from subterranean formations. As stated in the '243 patent, these so-called “solid stabilized emulsions” are such that “solid particles are the primary means, but not necessarily the only means, by which the films surrounding the internal phase droplets of an emulsion are maintained in a stable state under formation conditions for a sufficient time to use an emulsion as intended (e.g., enhance rate and/or amount of hydrocarbon production from a formation).”
The method of using a water-in-oil emulsion can be highly effective for certain oils and formations. However, the economics for such methods is typically very sensitive to the stability of the emulsion in situ. This is especially the case for the use of water-in-oil emulsions to displace heavy (viscous) oils. For a water-in-oil emulsion to have a viscosity sufficient to effectively displace a heavy oil, it requires a high concentration of emulsified water—typically >50 volume percent (vol %). Emulsion viscosity generally increases with increasing volume of the internal (emulsified) phase. If the viscosity of the emulsion is significantly less than that of the oil it is displacing, the emulsion will likely finger and channel through the native oil rather than uniformly displacing the native oil and thus lead to poor oil recovery. Thus, if the emulsion breaks down as it flows through the porous media of a reservoir, its viscosity and thus effectiveness will decrease.
A method for generating near-monodisperse droplets in an emulsion by shearing a previously generated emulsion has been disclosed. See T. G. MASON and J. BIBETTE, “Shear Rupturing of Droplets in Complex Fluids”, Langmuir, 13, 4600-4613, 1997; C. Mabille, et al., “Rheological and Shearing Conditions for the Preparation of Monodisperse Emulsions”, Langmuir, 16, 422-429, 2000. However, Mason is not directed to improving emulsion stability and fails to teach the steps of the disclosed method.
A method is disclosed in GB Patent No. 1,365,332 (the '332 patent) for improving the useful life of a cutting oil, which is essentially an oil-in-water emulsion used to lubricate the interface between a work piece and a machine tool. The method involves controlling bacterial infection in the cutting oil by continuously passing the cutting oil through a pasteurization heating system as is recycled through a flow circuit of the machine tool complex. A homogenizer stage may be placed in series with the pasteurization stage to regenerate the emulsion as it degrades through the system. The '332 patent does not disclose methods for improving emulsion stability other than by bacterial reduction nor for generating an emulsion which is not used in a continuous recycle system.
Accordingly, there is a need for a method to produce an emulsion with high stability that can be made economically, and especially is capable of performing under a wide range of subterranean formation conditions, including salinity, temperature, and permeability.
Other relevant material may be found in U.S. Pat. No. 3,149,669; U.S. Pat. No. 4,077,931; U.S. Pat. No. 4,232,739; U.S. Pat. No. 4,966,235; U.S. Pat. No. 4,983,319; and U.S. Provisional Application No. 61/070,156 titled “Viscous Oil Recovery Using Emulsions” filed on Mar. 20, 2008.